The invention relates to an object, comprising a substrate of a plastic material, as well as a plurality of optical layers. Such objects can be optical components such as lenses, prisms, reflectors.
A reflector comprises an essentially funnel-like base body which is reflectingly coated on its inner surface. Reflectors of this type are components of lamps of every possible type, such as, for example, spotlights. They consist of the materials glass, metal or plastic. The metal and plastic reflectors used in the automobile sector are provided with a metallic reflecting surface, which also reflects heat radiation.
In the zone of such reflectors considerable amounts of heat occur, which lead locally to temperatures of up to 200xc2x0 C. If the reflectors are made of glass, then no problems arise with respect to the temperatures mentioned.
Cold-light coated reflectors of glass substrates are found today, inter alia, in the general illumination field or in the stage lighting field. The cold-light coating there reflects only the visible radiation and transmits the heat radiation.
By reason of today""s spotlight geometries in motor vehicles with metallically coated reflectors, the infrared radiation reflected in this way leads to locally high thermal loads on components that follow in the beam path, such as, for example, lenses, covering panes and others. By reason of this load today these components as a rule have to be used as glass substrates and/or be protected by special coatings against the heat radiation.
In the automobile industry it is increasingly a matter of the saving of weight, since a weight reduction contributes to the saving of fuel. For this reason attempts are made insofar as possible to use plastic as a substitute material for other materials, for example, for steel. In the case of the glass reflector, however, so far this was not possible. The reflecting surface, namely, would have to be provided with a cold-light mirror. So far this has failed because it was not possible to fix such a cold-light mirror lastingly on a plastic.
There are also known processes for the production of objects comprising plastic material substrates with layers located on them. Such a process is described in DE 195 23 444 A1. There a plastic is provided with a protective layerxe2x80x94by means of a plasma-supported CVD process (PICVD process)xe2x80x94by an electric high-frequency discharge at a reduced gas pressure. There it is a matter of achieving as high as possible a viscosity of this layer for the avoidance of embrittlement.
DE 197 03 538 A1 describes a process for modifying surfaces of PMMA substrates. Here the substrate surface is provided with a protective layer. Therewith it is supposedly possible to achieve an improved adhesion of function layers subsequently to be applied. The applying of the protective layer mentioned presents an additional process step and means, therefore, additional expenditure and costs.
Further processes for applying thin layers to plastic substrates are described in DE 34 13 019 A1, EP 0 422 323 A1, DE 04 116 A1 and others. There it is also a matter, inter alia, of the adhesion of the layer that is applied to the substrate and, namely, by means of the CVD or PICVD process. DE 100 10 766 shows and describes a process and a device for coating in particular curved substrates, for example, of eyeglasses.
The processes used hitherto have not been satisfactory. The necessary adhesive strength was not achieved. On the contrary, with objects produced in such manner there is the danger of the detaching of the intermediate layer mentioned, and therewith of the entire layer pack. This can lead to the result that the object will become prematurely unusable.
Underlying the invention is the problem of giving a process with which a substrate of plastic can be lastingly and dependably provided with optical layers. In particular there is to be achieved herewith a permanent bonding of the layers to the substrate as well as of the layers among one another.
This problem is solved by the features of the independent claims.
The inventors have perceived that a lasting and dependable fixing of the alternating layers to the plastic as well as of the alternating layers among one another is attainable if the alternating layers are applied to the plastic substrate with a quite definitive process, namely by means of the so-called PICVC process (chemical plasma impulse vaporization).
In the hitherto used PICVC processes for applying layers to a substrate the interface of the substrate is disturbed or destroyed in its structure by the associated energy burden. This results in a reduction of the adhesiveness between substrate and adjoining layer. The inventors have perceived, accordingly, that the energy load which is associated with the plasma discharge must be minimized in order to enhance the adhesion. There it is a matters both of the quantity of the applied energy as well as also of the manner of its application. The admissible limit value of the load for achieving an adhesion sufficient in practice can be determined by experiment.
By the invention there can be utilized the advantages known per se of plastic material, in particular the low weight and the easy deformability, which plays a special role in automobile manufacture, as well as the avoidance of the hazards that are associated with the splintering of glass. Furthermore, if need be, it is possible to forgo the use of grounding layers such as grounding lacquers.
As plastic material there come into consideration high-performance plastics which are stable if possible up to a temperature of 100 degrees Celsius or above. Here again the PICVC process works out favorably, since the substrate temperature is kept relatively low. The layers generated are themselves hard and lasting.
As especially good as plastic materials, there have proved the following substances:
Cycloolefin polymers (COP)
Cycloolefin copolymers (COC)
Polymethyl methacrylate (PMMA), or derivatives thereof.
Several properties of the COP and COC plastics make components of COP-COC plastics with optical interference layer systems especially well suited for their use as optical components with or without treatment or coating:
(i) High transparency with visible and near (nahmen) infrared spectral range, low double refraction
low light losses and low heating-up by heat absorption
(ii) High thermal form stability
(iii) Components made of COP/COC can be produced with a precision to that of glass
(iv) Barrier effect especially against water vapor
advantageous for good adhesion of layer systems
By reason of the properties mentioned COP and COC plastics can in many cases replace glass substrates and they therewith open up new possibilities in the design and layout of optical systems. In combination with specially adapted interference layer systems, high-quality optical components are producible.
With the PICVD process one has in hand a coating technology which, besides for the coating of glass and the like, is also excellently suited to the coating of plastic. An example of application for this is a plastic reflector with cold-light reflection, such as is of interest above all for the automobile industry. Cold-light coated reflectors of glass substrates are found today inter alia in the general illumination sector or in the stage-lighting sector. The cold-light coating there reflects only the visible radiation and transmits the heat radiation.
By reason of today""s headlight geometries in motor vehicles with metal coated reflectors the infrared radiation in this manner leads to high thermal loads on components that follow in the beam path, such as, for example, lenses, covering panes and the like. Because of this load, today these components must be installed as a rule as glass substrates and/or be protected against heat radiation by special coatings.
In the automobile industry it is increasingly a matter of the saving of weight, since a weight reduction contributes to the saving of fuel. For this reason attempts are made insofar as possible to use plastic material as a substitute for other materials, for example, for steel. In the case of the glass reflectors, however, this was hitherto not possible. The reflecting surface, namely, would have to be provided with a cold-light mirror. So far this has failed because it was not possible to fix such a cold-light mirror lastingly onto a plastic material.
According to the invention, therefore, the base body of the reflector is made of plastic material, with all its advantages which consist above all in a low specific weight and a low deformability. Also, the problem of splintering protection is eliminated.
Further, according to the invention the reflecting surface is constructed as a cold-light mirror. It consists suitably of a plurality of layers with different refraction coefficients. This coating reflects the visible radiation and lets the infrared radiation be transmitted (cold-light mirror principle).
According to the invention it has proved that the alternating layers are fixable dependably and durably to the plastic material, and, namely, if they are applied with quite definite processes to the plastic substrate, namely, by means of the plasma-impulse process or by means of chemical vaporization or by means of phase-impulse chemical vaporization. In a preferred form of the phase-impulse chemical vaporization process, the total duration of the plasma action amounts to at least {fraction (1/1000)} of the total action-free time span and is at most equal to this time span. The impulse action of the plasma lasts between 0.1 and 10 ms and preferably between 0.5 and 5 ms at a coating rate of the plasma action per time unit and per surface unit  greater than 10 nanometers/min. and preferably greater than 100 nanometers/min.
Because of the temperature load only high-performance plastics come into considerations that are stable at least up to 100xc2x0 C., and better higher. In order to apply hard lasting cold-light layer systems, as a rule high thermal loads on the substrate are required, which a plastic reflector does not endure without damage. Here the PICVD process offers itself, which as a pulsed process brings about only a slight energy charge into the substrate and thus keeps the substrate temperature low. On the other hand, in the pulses high plastic energy densities are generated that produce strong and durable layers. The coating is, as mentioned, a multiple coating. The individual layers are optical functional layers such as, for example, an anti-reflex layer, but also multi-function layers with special mechanical properties such as for example a hydrophobe cover layer, the so-called cleaning layer, which can easily be cleaned mechanically. Also layers with special optical and other properties come into consideration. Between the functional layers and the substrate there is generally applied a so-called bonding agent layer, here quite generally called xe2x80x9cinterlayerxe2x80x9d. This has a decisive importance for the usefulness of the entire product. It is responsible, namely, for the adhesion of the entire coating pack to the substrate. The interlayer can be a single homogeneous layer. It can also be a multiplex layer. There the individual layers form a gradient proceeding from a layer that follows immediately upon the substrate and, as an organicum, is identical or similar to a layer that is close to the next-following functional layer and is identical or similar to this as an inorganicum.
Further, the invention is usable with every type of coating. The coating properties can be controlled by the raw materials used as well as by the operating parameters of the installation. The layer properties can vary within wide limits. The interlayer will in general be a so-called gradient layer, which is composed of several layers, and in which the layer near the substrate is essentially identical to the substrate, and the layer remote from the substrate is essentially identical to the functional layers.
The invention can also be used with application of the so-called remote-PICVD process. Here, as is well known, plasma space and coating space are separate from one another. From the plasma space excited species are led into the coating space.